1. Field of the Invention
This invention lies in the field of determination of subsurface earth structure resulting from the application of pressurized fluid at one or more known points in the earth, by the determination of the change in surface contour of the earth above the points of application.
More particularly, it concerns the placement on the surface of instruments known as tiltmeters, which are sensitive to the angle of tilt of the earth, after being positioned rigidly in the earth, and leveled. Any change of subsurface structure, such as upward or lateral movement of parts of the subsurface, will be reflected in the surface contour, which can be detected by the tiltmeters. The information of changes in tilt as a function of changes in fluid flow rate and pressure, will provide the basis for determination of subsurface structural change.
Still more particularly, it concerns the interpretation of the surface measurements to provide indication of the azimuth of a deep vertical fracture, and an estimate of its length.
This application is concerned primarily with deep vertical fractures in the earth.
2. Description of the Prior Art
In the oil industry for the last 20 or more years, great amounts of money and effort have been devoted to a process known as hydrofracture, which has been and is now being, employed for the purpose of creating large fractures in the subsurface geological formations. These fractures are propped open so that the elasticity of the rocks and their weight will not cause the fracture to be closed up when the fluid pressure is released. By this means, continuous openings are provided for the flow of liquid and gaseous hydrocarbons through the fracture to the well bore. Thus the hydrocarbons can be recovered more readily than was possible before the fracture was initiated.
However, in spite of the hundreds of thousands of fractures carried out over this period, and in spite of the great effort devoted to finding a method for positively determining the position, the attitude, direction, and extent of a fracture that may have been created in the earth, nothing has resulted to provide such answers. In spite of the fact that many hundreds of thousands of dollars are spent on a single fracture, there is no known way of determining the details of the fracture, if any, other than after an expanded period of testing the well, to determine whether its productivity has increased or not. It the productivity is increased, it would indicate the presence of a fracture. However, there is still no way of determining the size, extent, and direction of the fracture.